回答編集履歴
1
もっと上手くできるかも
    
        answer	
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         @@ -19,4 +19,69 @@ 
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              $('#hoge').animate({scrollTop: $('#hoge')[0].scrollHeight}, 'fast');
         
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            }
         
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            ```
         
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            JavaScriptのタイミングは適当に変えてください。
         
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            JavaScriptのタイミングは適当に変えてください。
         
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            ### 追記
         
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            比較演算子の調整が必要かも
         
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            ```HTML
         
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            <body>
         
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              <div class="container area bg-light" id=hoge>
         
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                <div id=messages>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">hoge</div>
         
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                  <div class="message">これをbottom: 0の位置にくるようにしたい</div>
         
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                </div>
         
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              </div>
         
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            </body>
         
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            ```
         
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            ```CSS
         
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            .area {
         
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              height: 50vh;
         
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              overflow-y: scroll
         
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            }
         
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            ```
         
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            ```JavaScript
         
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            window.onload = function() {
         
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              var aH = window.innerHeight / 2;
         
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              var mH = $('#messages').outerHeight();
         
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              if (mH<aH) {
         
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                var offset = $('#messages').offset({top:aH-mH});
         
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              } else {
         
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                $('#hoge').animate({scrollTop: $('#hoge')[0].scrollHeight}, 'fast');
         
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              }
         
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            }
         
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            ```
         
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            両方要jQuery
         
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            思いつき等々で書いたのでもっといい書き方あるかもしれません。
         
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